In oceanic biogeochemistry, the transport of organic carbon via sedimentation down to the deep water and finally to the sediments of the ocean is referred to as the biological carbon dioxide pump. Initially, carbon dioxide is taken up by phytoplankton during photosynthesis and converted into organic carbon. However, most of this carbon is released again during respiration, either by the phytoplankton itself or by zooplankton and bacteria, which feed on phytoplankton or its dead remains.
Near-surface respiration cycles carbon dioxide back into the water, from where it can return to the atmosphere. Only a minute fraction of the organic matter produced in the surface ocean sinks down to waters below the permanent thermocline. These waters of great oceanic depth do not get into contact with the atmosphere on a seasonal basis. Only this fraction of the primary production of organic carbon acts as a drain on atmospheric carbon dioxide and can be termed the CO2 pump.